Processes controlling 234U and 238U isotope fractionation and helium in the groundwater of the St. Lawrence Lowlands, Quebec: The potential role of natural rock fracturing

Méjean, Pauline; Pinti, Daniele L.; Larocque, Marie; Ghaleb, Bassam; Meyzonnat, Guillaume et Gagné, Sylvain (2016). « Processes controlling 234U and 238U isotope fractionation and helium in the groundwater of the St. Lawrence Lowlands, Quebec: The potential role of natural rock fracturing ». Applied Geochemistry, 66, pp. 198-209.

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Résumé

The goal of this study is to explain the origin of 234U–238U fractionation in groundwater from sedimentary aquifers of the St. Lawrence Lowlands (Quebec, Canada), and its relationship with 3He/4He ratios, to gain insight regarding the evolution of groundwater in the region. (234U/238U) activity ratios, or (234U/238U)act, were measured in 23 groundwater samples from shallow Quaternary unconsolidated sediments and from the deeper fractured regional aquifer of the Becancour River watershed. The lowest (234U/238U)act, 1.14 ± 0.01, was measured in Ca–HCO3-type freshwater from the Quaternary Shallower Aquifer, where bulk dissolution of the carbonate allows U to migrate into water with little 234U–238U isotopic fractionation. The (234U/238U)act increases to 6.07 ± 0.14 in Na–HCO3–Cl-type groundwater. Preferential migration of 234U into water by α-recoil is the underlying process responsible for this isotopic fractionation. An inverse relationship between (234U/238U)act and 3He/4He ratios has been observed. This relationship reflects the mixing of newly recharged water, with (234U/238U)act close to the secular equilibrium and containing atmospheric/tritiogenic helium, and mildly-mineralized older water (14C ages of 6.6 kyrs), with (234U/238U)act of ≥6.07 and large amounts of radiogenic 4He, in excess of the steady-state amount produced in situ. The simultaneous fractionation of (234U/238U)act and the addition of excess 4He could be locally controlled by stress-induced rock fracturing. This process increases the surface area of the aquifer matrix exposed to pore water, from which produced 4He and 234U can be released by α-recoil and diffusion. This process would also facilitate the release of radiogenic helium at rates greater than those supported by steady-state U–Th production in the rock. Consequently, sources internal to the aquifers could cause the radiogenic 4He excesses measured in groundwater.

Type:	Article de revue scientifique
Informations complémentaires:	© <2016>. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/
Mots-clés ou Sujets:	(234U/238U) activity ratio, Helium isotopes, Fractured aquifers, α-recoil, 4He excess, St. Lawrence Lowlands
Unité d'appartenance:	Centres institutionnels > Centre de recherche en géochimie et géodynamique (GEOTOP) Faculté des sciences > Département des sciences de la Terre et de l'atmosphère
Déposé par:	Marie Larocque
Date de dépôt:	21 mars 2016 14:23
Dernière modification:	01 mai 2018 23:10
Adresse URL :	http://archipel.uqam.ca/id/eprint/7993

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